A cost effective method to control an elevator door system can be achieved by using a velocity closed loop control and a look-up table in a computer for variable frequency control of an AC motor. The door system is designed for high performance.
The low cost realization of such a door system is done with an eight-bit microprocessor. To achieve constant dynamics of the closed loop control in all the operating regions of the door systems, that is, normal opening/closing runs or reversal, the velocity controller is of the adaptive type. FIG. 1 shows the block diagram of the control system.
The normal run consists of a velocity profile for the full doorway. If the door movement should change from closing to opening or vice versa, the door has to be stopped first, and then it can change the run direction with another different profile designed for the corresponding short run. Stopping of a moving door and reversal of the direction of movement of the door is generally known in the elevator art as "reversal." The stopping must not show oscillations. FIG. 2 shows the different profiles resulting from the given door commands. Additionally, the variation of the controller or gain is given depending on the velocity profile.
The closed loop control should have the same dynamic behavior for the starting phase with each velocity profile, that is, the long profile run for the complete doorway or the short run, when reversal occurs. FIG. 3 shows the mechanical assembly of such a door. The needed force to start the door movement depends on the actual position of wheel A in FIG. 3.
Depending on the operating point, the non-linear characteristics of the mechanical assembly change when the performance changes. To assure good performance in all operating points, an adapted velocity control is necessary in order to track velocity dictation, especially when starting with opening/closing at low speed. Several techniques are known to control an AC motor like the field-oriented control. Generally, the relationship between a dictated stator frequency and reference stator voltage in a variable frequency induction motor drive is a ramp function until a point is reached at which an increase in dictated stator frequency yields the same reference stator voltage. The control system consists of a velocity control with an inner current control loop. In this case, a high performance microprocessor/controller or a digital signal processor (DSP) is necessary to fulfill the requirements. In addition, two current sensors are needed for the torque control. The realization of such a control system results in high cost.